Devices familiarly referred to as pH electrode sensors are widely used to sense the acidity of (i.e., hydrogen ion concentration in) many types of fluids. Typically, a pH sensor consists of two electrodes, a pH electrode and a reference electrode, which are together connected to an instrument that senses the difference in the electrical potential between the electrodes. Ordinarily, the reference electrode is located in the sensor coaxially within a chamber coaxially surrounding the pH electrode, and a reference solution having a known electrolyte concentration surrounds the reference electrode. See, for example, U.S. Pat. No. 3,741,884.
To measure the pH of a specimen fluid, the sensor structure, including the pH electrode and reference electrode, is immersed in the fluid, with the pH electrode in direct contact with the fluid. The reference electrode, on the other hand, is in electrochemical communication with the specimen fluid through a salt bridge. In accordance with well-known electrochemical principles, the reference electrode generates a stable potential, regardless of the pH of the specimen fluid. The pH electrode, on the other hand, generates a potential that varies as a function of the pH. The potential difference between the electrodes is measured and is instrumentally correlated to pH.
The stability of the reference electrode's potential is critical. If that potential changes over time, or if it changes in response to pH, an accurate pH reading will not be obtained.
To establish an electrical contact between the reference electrode and the specimen fluid, the chamber that holds the reference electrode is typically surrounded by a semi-porous material which is impregnated with a salt, e.g., potassium chloride. The effect of the salt is to electrically "bridge" the space between the specimen fluid and reference electrode.
Not surprisingly, many structures have been introduced which provide for electrochemical contact between the reference solution and the specimen fluid by means of a "salt bridge". For example, U.S. Pat. No. 3,440,525 to Cardeiro discloses a salt bridge comprising a porous plug of wood saturated with electrolyte. One end of the plug contacts the specimen liquid, and the other is in the electrolyte solution surrounding the reference electrode. Numerous pathways exist through the many capillaries in the wood so that the salt bridge solution can make electrochemical contact with the specimen fluid. Thus, if some of the capillaries become clogged, electrochemical contact between the reference solution and specimen fluid can still be maintained through other of the capillaries.
Unfortunately, the many capillaries of the Cardeiro plug establish the possibility of a substantial flow of contaminating ions through the plug. Because the osmotic pressure differential between the specimen fluid (which typically has a low osmolality) and the electrolyte solution, the specimen fluid will attempt to migrate into the salt bridge and into the electrolyte solution. Contaminating ions can reach the electrolyte solution and the reference electrode, causing changes in the reference potential and corresponding errors or instability in pH measurement.
To overcome the problems associated with the Cardeiro plug, U.S. Pat. No. 4,112,352, which issued to Barben, discloses several cylindrical wood plugs which are positioned end-to-end and which are impregnated with a salt solution, with a bore being centrally formed in each plug. A pH electrode is positioned in the central bore. Also, a cavity is formed in the end of one of the two end plugs, and a reference electrode is disposed in the cavity. Wood dowel pins connect each plug with the immediately adjacent plugs, and successive dowel pins are longitudinally offset from each other. This provides an interlocking, multi-piece, nonintegral plug which serves as a salt bridge.
In accordance with the Barben invention, each end surface of each piece of the plug which abuts the end surface of another piece is coated with a non-porous substance. Consequently, the only way for ion migration to occur through the capillaries of the wood plugs is for the ions to traverse the tortuous path in sequence through a first plug, then through a first dowel pin, then through a second plug, then through a second dowel pin, and so forth. In this way, ion migration is slowed, delaying the ultimate contamination of the reference electrode.
Unfortunately, the Barben device requires the forming and assembly of several plugs and dowels. This is relatively labor intensive and thus costly. Further, after fabrication of the plug, impregnation with the electrolyte solution is difficult and time consuming.
Accordingly, it is an object of the present invention to provide a pH sensor which permits a low impedance electrochemical connection between the reference cell of the sensor and a specimen fluid. Another object of the present invention is to provide a pH sensor which to a much greater degree retards flow of contaminating ions through the salt bridge to the reference electrode by immobilizing the ions in ion traps. This, in turn, significantly prolongs the service life of the sensor. A further object of the present invention is to provide a pH sensor which is easy to manufacture.